1. Field of the Invention
The present invention relates to a high fatigue strength gear, well-suited for use on an engine crank shaft.
2. Description of Background Art
Various high fatigue strength gears are known in the art. These gears are made from a soft nitriding steel, such as a low or medium carbon steel containing Al, Cr, and the like. These types of steel are specified as, for example, JIS SACM645.
Such soft nitrating steels cannot achieve an acceptable fatigue strength necessary for a gear, simply by virtue of soft nitriding, alone. Therefore, the steel is quenched and tempered to improve its inner hardness, in other words, its internal strength.
Since the soft nitriding is applied to a semi-finished gear after being mechanically worked. The hardness is increased during the quenching and tempering. This has the adverse result of limiting the mechanical workability of the steel. As a result, the fatigue strength of the gear, particularly, the bending fatigue strength of the dedendum of the gear is impaired. That is, the gear produced in this way is inferior in bending fatigue strength to a gear subjected to carburizing.